FIG. 1 shows a cross-sectional view of a conventional planer-type photodiode comprising semiconductor layers produced by means of selective epitaxial growth, being disclosed in Japanese Patent Application No. 7-52700. After forming a trench on a N--Si substrate 1, a SiO.sub.2 layer 2 is grown on the side wall of the trench and a P.sup.- --Si epitaxial layer 6 (a light-absorbing layer) and a P.sup.+ --Si epitaxial layer 5 are grown selectively and continuously.
The impurity concentration of the light-absorbing layer 6 is selected to be less than 1E15 cm.sup.-3 in order to make the depletion layer extend to the light-absorbing layer 6, when a reverse bias voltage is applied to a PN-junction. If electric charges are generated in the depleted light-absorbing layer 6 by external optical energy, a photo-electric current arises. On a boundary surface between the light-absorbing layer 6 and the SiO.sub.2 layer 2, both shown in FIG. 1, state of crystallizations is disordered, and considerable numbers of surface states are produced thereon. If the reverse bias voltage is applied to the PN-junction and the depletion layer is extended around the aforementioned boundary surface, leakage currents flow via the surface states, and thereby characteristic of the photodiode is deteriorated.
The second example of the conventional photodiode shown in FIG. 2, which brings the aforementioned disadvantages to a settlement, is disclosed in Japanese Patent Kokai No. 5-291605, and has a mesa-shaped cross-section, which has been frequently used in a photodiode comprising compound-type semiconductor layers. In this example, a light-absorbing layer, in which a depletion layer is extended, is a N.sup.- -InGaAs layer 24. Since there are many surface states on the mesa-shaped and etched surfaces of the light-absorbing layer 24, P-InP layers 22 are formed on the both side surfaces of the mesa in order to suppress extension of the depletion layers thereto. Then, P-impurity diffusion layers 25 are formed by impurity diffusion in thermal treatment. In this structure, even when a reverse bias voltage is applied to the PN-junction, the depletion layer is hardly extended into the P-impurity diffusion layers 25, the number of the surface states is decreased therein, and the leakage current can be reduced, if the impurity concentration of the P-regions 22 is higher than that of the N.sup.- -InGaAs light-absorbing layer 24 by a factor of several hundreds.
FIG. 3 is a cross-sectional view of the third example of a conventional photodiode of diffusion type, which is often used in Si photodiodes, being disclosed in Japanese Patent Kokai No. 2-291180. In this example, the light-absorbing layer, in which the depletion layer extends, is formed in a N.sup.- --Si substrate 31. If the depletion layer extends as shown in FIG. 3, the depletion layer extends along the surface of the substrate, which comprises comparatively large numbers of surface states, and a leakage current increases. From a view point of the aforementioned situation, N.sup.+ -diffusion layers 33 are formed on the surface of the N.sup.- -substrate in order to suppress extension of the depletion layer near the surface of the substrate, and thereby increase of the leakage current can be prevented.
In the first example of the conventional photodiodes shown in FIG. 1, the leakage current is liable to flow via the surface states on the boundary surface between the light-absorbing layer 6 and the SiO.sub.2 layer 2. The reason is that the state of crystallization of the boundary surface between the light-absorbing layer 6 and the SiO.sub.2 layer 2 is disordered, because the light-absorbing layer 6 is formed by process of selective epitaxial growth.
The second example of the conventional semiconductor photodiodes shown in FIG. 2 can be applied only to the mesa-type photodiode, and cannot be applied to the buried-type photodiode shown in FIG. 1. Moreover, since the capacitance of the PN-junction increases in the structure shown in FIG. 2, it becomes obstruction for achieving a photodiode suited for high bit-rate operation. The reason is that the epitaxial growth of the semiconductor layers with an opposite conductivity type to that of the light-absorbing layer is necessary on the both sides of the mesa, after the light-absorbing layer is grown.
The method for decreasing the leakage current shown in FIG. 3 used in the third conventional photodiode is applicable only to the photodiode of diffusion type, in which the light-absorbing layer is formed in the semiconductor substrate, but is not applicable to the photodiode of buried type shown in FIG. 1. The reason is that the N.sup.+ -impurity diffusion layer is formed on the surface of the substrate near the PN-junction.